The NOx sensor including a sensor element formed of an oxygen-ion conductive solid electrolyte ceramic such as zirconia (ZrO2) has been conventionally known as a device for measuring a NOx concentration in a measurement gas such as an exhaust gas and a combustion gas in an internal combustion engine, usually a car engine (for example, see Patent Document 1). Such a NOx sensor obtains the concentration of the NOx gas by utilizing a fact that upon decomposition of the NOx gas at a measuring electrode, the amount oxygen ions generated at that time is proportional to the amount of a current (also referred to as a NOx signal) flowing through the measuring electrode and a reference electrode.
In the gas sensor according to Patent Document 1, a ceramic sensor element is fixed by a plurality of ceramic supporters made of ceramic insulator and ceramic powder compacts each filled between the ceramic supporters, such as talc, in a metallic housing and a hollow portion of a cylindrical inner tube welded and fixed to the housing, and is hermetically sealed with the powder compacts.
The step of assembling a gas sensor disclosed in Patent Document 1 includes the step of sequentially fitting through holes, which are provided at the axis center positions of the ceramic supporters and powder compacts, with the sensor element to annularly mount the members to the sensor element, and the step of fitting the intermediate assembly product, which is obtained by annularly mounting the ceramic supporters and powder compacts to the sensor element, with the cylindrical inner tube to annularly mounting the inner tube to the intermediate assembly product.
In the fitting step as described above, if a part serving as an axis warps and a clearance between fitting portions of the respective parts is narrow, the part serving as an axis may interfere with the fitting parts, which does not allow the parts to be fitted in a predetermined manner.
For the gas sensor as disclosed in Patent Document 1, the airtightness between the measurement gas space and reference gas space needs to be secured for preventing the measurement gas from flowing into the reference gas space, while the size of the gas sensor is desired to be minimized because it is provided in an exhaust pipe of the internal combustion engine of, for example, a car. This requires to reduce the inside diameter of the inner tube as well as the sizes of the ceramic supporters and powder compacts and to set a dimensional tolerance smaller than that of a typical fitting part. In other words, the assembly to be made in a situation in which a sufficient clearance is not secured is required. This may result in a case where the ceramic supporter or powder compact interferes with the sensor element or the intermediate assembly product interferes with the inner tube, hindering smooth assembly. In particular, such an interference tends to occur in automation of the above-mentioned assembly step. Also, even in a case where a sensor element whose warping poses no problem in element characteristics is used in assembly, the assembly may be made poorly due to the above-mentioned interference that has occurred.
Setting of a dimension and tolerance for sufficiently securing a clearance between fitting parts may reduce such problems, which is not desirably employed from viewpoints of hermitic sealing and size reduction of an element as described above.